Field of the Invention
This invention relates generally to solar heating systems of a kind whose normal operation involves circulation of a heat transport liquid through a solar collector and a storage tank to store solar thermal energy in the tank. The invention relates more particularly to a redundant freeze protection method and system for such solar heating systems.
Prior Art
As will appear from the ensuing description, the present freeze protection invention may be utilized in a variety of solar heating systems of the class described. The invention, however, is intended primarily for use in a solar water heating system and will be described in this context.
Simply stated, a solar heating system of the kind to which this invention pertains comprises a solar collector, a storage tank, and means for circulating a heat transport liquid through the collector and tank to transfer solar thermal energy from the collector to the tank. In a solar water heating system, a hot water storage tank contains water which is heated by the solar energy transferred to the tank.
There are two general types of solar water heating systems, commonly referred to as "direct" and "indirect" types. In the direct system, potable water from the supply main constitutes the heat transport liquid which circulates through the solar collector and the hot water tank to the point of use and is consumed by the user. In contrast, in the indirect system, the heat transport liquid is a separate liquid which circulates through a closed loop including the solar collector and a heat exchanger through which thermal energy is transferred to the water in the tank. As will appear from the ensuing description, the present freeze protection invention is utilized in the direct type of solar water heating system.
The prior art is replete with a vast assortment of solar water heating systems of the character described. One such system is described in the earlier referenced co-pending application. Other systems are described in U.S. Pat. Nos. 3,812,872; 3,989,032; 4,015,962; 4,019,495.
Solar water heating systems of the kind to which this invention pertains are subject to one problem to which this invention is addressed, i.e., freezing of the heat transport liquid in the solar collector. This problem arises, of course, due to the fact that the solar collector is exposed directly to the atmosphere and hence the liquid in the collector is subject to freezing when the air temperature approaches, drops to, or is below freezing.
A variety of freeze protection methods and systems have been devised to either prevent or accomodate freezing of the collector liquid. One common method of freeze protection, for example, involves circulation of liquid through the storage tank and solar collector to heat and thereby prevent freezing of liquid in the collector. The referenced co-pending application embodies this type of freeze protection. While satisfactory, at least to a limited extent, this method of freeze protection is subject to the disadvantage that it consumes or wastes the stored thermal energy. Depending upon the duration of the freeze conditions, the air temperature, and the total quantity of thermal energy stored, the stored energy may be totally depleted and, if not augmented by additional energy input, freezing of the collector liquid may occur.
The patents listed earlier teach a second method of freeze protection involving draining of liquid from the solar collector. This avoids the stored energy depletion problem associated with the collector heating method of freeze protection, but introduces another potential problem, which is the possibility of failure of the collector drainage means to drain the solar collector.
Accordingly, neither of the above described freeze protection methods is totally satisfactory. An improved method of freeze protection is thus needed.